Project #2: This project focuses on the effects of combination antiretroviral treatment (cART) on the central nervous system (CNS) infection and its consequences. It extends the findings of Project 1 related to cerebrospinal fluid (CSF) compartmentalization and T-lymphocyte and macrophage- tropism by examining their pathobiological consequences and influence on treatment responses. On the basis of previous studies, we postulate three types of CNS infection that differ in their location and tropism of the virus. Using CSF immune and neural biomarkers, neuropsychological test performance, this project proposes a series of studies to further characterize these three of CSF infection types. The project proposes three specific aims that examine, in turn, the effect of compartmentalization and tropism on: the pretreatment state, the treatment response kinetics and the longer term outcomes. Thus, the Aim 1 assesses the effect of pretreatment CSF viral compartmentalization and tropism on the other major baseline variables of interest that include: CSF immune activation and chemokine biomarkers, CSF neural injury biomarkers, and neuropsychological performance. Aim 2 examines the effect of pre-treatment compartmentalization and tropism on the rate and magnitude of CSF HIV response to treatment compared to that of plasma, as well as the rate and magnitude of the reduction of intrathecal immune and chemokine responses, 'normalization'of CSF neural biomarkers, and improvement in neuropsychological performance. Aim 3 assesses the effects of baseline compartmentalization and tropism on subsequent CSF viral suppression and 'CSF viral 'escape'(or 'discordant CSF response') after >1 year of treatment, and characterizes the CSF immune/Inflammatory and neural biomarker profiles and neuropsychological test performance in subjects with CSF viral escape compared to those with CSF suppression.